Unraveling Ambimodal Periselectivity in Pentafulvene–Radialene Cycloadditions: Interplay of Geometry, Charge Delocalization, and Reaction Dynamics

The
ambimodal cycloaddition between pentafulvene and a homologous
series of [n]radialenes (n = 3–6)
was systematically investigated using density functional theory, DLPNO–CCSD(T)
calculations, and quasi-classical molecular dynamics (QCMD) simulations
to elucidate the geometric and electronic origins of periselectivity.
Despite their intrinsic instability, these π-rich systems enable
the formation of multiple products through bifurcated potential energy
surfaces (PESs). Two distinct ambimodal transition states were identified,
corresponding to bispericyclic (exo) and trispericyclic (endo) pathways.
The reaction revealing [5]-radialene exhibits the most pronounced
ambimodal character, driven by favorable exocyclic π-overlap,
extensive charge delocalization, and the lowest activation barrier.
Intrinsic bond orbital and interaction region indicator analyses reveal
asynchronous σ-bond formation and progressive orbital reorganization
that govern dynamic branching behavior. QCMD simulations further confirm
that product distributions are dictated by PES topography rather than
static transition-state energetics, underscoring the significance
of dynamic effects in determining selectivity. This comprehensive
study establishes a unified mechanistic framework linking geometry
and electronic delocalization to dynamic selectivity control in cross-conjugated
π-systems, offering predictive insights into ambimodal bis-
and trispericyclic reactivity with potential implications for synthetic
and biosynthetic design.